The egg processing industry discards over 600,000 tons of eggshells per year, after breaking the egg and removing the egg white and yolk. Eggshells are generally thought of as a waste product, however if the inorganic and organic components are separated, commercial materials can be obtained and utilized in a variety of applications. Large amounts of eggshell waste or by-product are generated annually and the majority ends up in landfills. Therefore the process of isolating the valuable products is of interest to keep the eggshell by-product out of landfill and to turn a low value material into a high value one.
There are two major components in eggshells: the membrane and the shell. The membrane consists mainly of organic material, including collagen and amino acids. These materials can be separated from the shell and used in the medical and cosmetic industries, among others. The inorganic component of eggshell consists of calcium carbonate with small amounts of magnesium carbonate and calcium phosphate. The majority of this portion is calcium carbonate (CaCO3), which has a variety of applications. Many of these applications require the calcium carbonate product to be of a high purity, particularly where the calcium carbonate is to be used for human consumption.
There is variation across the egg processing industry of the process for discarding the shell. Some egg ‘breaking’ companies will dump and discard the remaining shell by-product immediately after the removal of the liquid egg. This eggshell by-product will still have remnants of wet material. Other breaking companies will first run the by-product through a centrifuge-type apparatus, for the purpose of removing the bulk of the remaining liquid egg, which will aid in reducing cost to discard the by-product.
Approaches to separating shells from membranes known in the prior art have limitations when considering the purity of the final calcium carbonate product, cost effectiveness of the approach, and scalability of the approach. Several prior art approaches are focussed on the purity and processing of the membrane by-product rather than the calcium carbonate by-product.
A number of approaches have been proposed where the separation is purely mechanical in nature, which inherently results in a relatively impure final calcium carbonate product with organic impurities. For example, U.S. Pat. No. 6,176,376 and U.S. Pat. No. 7,007,806 of MacNeil uses agitation in a liquid to create separation of the membrane and eggshell by relying on the differences in bulk density between the membrane and eggshell portions. The eggshell settles to the bottom of the tank while the membranes remain suspended in liquid. U.S. Pat. No. 6,649,203 of Thoroski describes an approach with centrifuging, washing, centrifuging, drying, and milling stages followed by a pneumatic membrane removal. In the pneumatic removal step, the membrane and eggshell fall through a suction flow which diverts the relatively light membrane but not the eggshell.
An approach that combines mechanical and chemical approaches may be seen in U.S. Publication No. 2006/0159816 and U.S. Pat. No. 7,954,733 of Vlad. This approach uses cavitation in a fluid tank (a mechanical step) to separate membrane from eggshell. Optionally, the membranes may be dried. Acetic acid may then be used to extract certain polypeptides from the membrane. However, this approach, and specifically the chemical extraction, is focused upon separation and treatment of the membrane materials, not the shells which are the source of calcium carbonate. In respect of the shells, they are again separated through a mechanical separation process only, and will have relatively high levels of organic impurities.
U.S. Pat. No. 7,597,280 of Floh describes a system wherein a slurry of finely ground shell and membrane is introduced into a separation tank with an upward flow and a number of overlapping vanes. The membrane is borne upwards and withdrawn by the vanes; the shell falls to the bottom of the tank and is removed. The membrane is dewatered (using protein dewatering) and dried. Again, the focus of this approach is on the production of the membrane as a product. In respect of the shells, they are separated through a mechanical separation process only, and will have relatively high levels of organic impurities. This approach is also quite costly.